1. Field of the Invention
The present invention relates to an inkjet ink capable of providing high print quality on plain paper, recycled paper, or coated paper. The present invention relates to an inkjet ink capable of ensuring high ejection stability and providing high print quality on plain paper, recycled paper, or coated paper.
2. Description of the Related Art
Inkjet recording is a method in which ink is ejected as droplets from minute nozzles to record characters and patterns on the surface of a recording medium. The inkjet recording systems used include methods in which electric signals are converted to mechanical signals with the aid of an electrostrictive element, and ink stored in a nozzle head portion is continuously ejected to record characters or symbols on the surface of a recording medium; and methods in which ink stored in a nozzle head portion is foamed by the rapid heating of the part that is the closest to the ejection portion, discontinuously ejected by foaming-induced volumetric expansion, and used to record characters or symbols on the surface of a recording medium.
The ink used for such inkjet recording must ensure good print drying, no print bleeding, uniform printing on the surface of the recording medium, no blending of colors in the case of multicolor printing, and other characteristics. A particular problem in this case is that bleeding can be easily caused by fibers that have different permeability levels when paper is used as the recording medium.
In conventional inks for inkjet recording, glycol ether is often used as a wetting agent, as in Japanese Patent Publication No. H2-2907; or as a water-soluble organic solvent, as in Japanese Patent Publication No. H1-15542; or as a dyeing and dissolution accelerator, as in Japanese Patent Publication No. H2-3837.
To improve permeability, studies have been conducted into adding diethylene glycol monobutyl ether, as in U.S. Pat. No. 5,156,675; or adding Surfynol 465 (manufactured by Nisshin Chemical), a surfactant based on acetylene glycol, as in U.S. Pat. No. 5,183,502; or adding both the diethylene glycol monobutyl ether and Surfynol 465, as in U.S. Pat. No. 5,196,056. Diethylene glycol mono-n-butyl ether, which is called butyl carbitol, is described, for example, in U.S. Pat. No. 3,291,580. Alternatively, using diethylene glycol ethers for the ink, and other issues are studied in U.S. Pat. No. 2,083,372.
In cases in which pigments are used, many studies have been conducted and practical implementations introduced with the primary goal of reducing permeability and making the paper surface less wettable by ink to ensure adequate print quality. Alternatively, combinations of glycol ether and a pigment include examples in which triethylene glycol monomethyl ether is used for the pigment, as in Japanese Patent Application Laid-open No. S56-147861.
The prior art is disadvantageous, however, in that the ink permeability of paper is often insufficient and that the methods for reducing the wetting of the paper surface still allow the ink to spread on plain paper, and the widely used recycled paper in particular, and require some time for the printed matter to dry, for which reason the ink on the printed paper is slow to dry and cannot be quickly overlaid when continuous printing is performed. In addition, recycled paper is obtained by mixing components of various types of paper and is an aggregate of materials that have different permeability rates, with bleeding caused by the differences between these permeability rates. Systems and the like for heating the paper are commonly contemplated in order to reduce such bleeding. Heating paper or another print object during printing is disadvantageous, however, in that time is required to bring the heating unit in the device to a specific temperature, the device main body consumes more power, and the paper or other print object may be damaged.
Inks in which pigments are used are also disadvantageous in that when printing is performed on paper or other material having an ordinary sizing agent as the recording medium, the pigment remains on the surface of the paper or the like, and scratch resistance is impaired unless the ink is endowed with a measure of permeability. However, keeping permeability at an intermediate level limits the number of paper types on which uniform printing can be performed, and tends to reduce the quality of printed images.
In addition, methyl ethers such as those employed in Japanese Patent Application Laid-open No. S56-147861 are often used as glycol ethers for controlling permeability in inks obtained using pigments. Furthermore, nothing is as yet known about pigment-based inks for inkjet recording in which permeability can be improved with butyl ethers and printing can be accomplished with reduced spreading on a large number of paper types.
Among the characteristics required of the ink compositions used in inkjet printers or the like, the most important are (1) providing high-quality printed matter, (2) ensuring rapid drying on recording media, and (3) increasing reliability during recording.
The following items are taken into account in a comprehensive manner in relation to the requirement (1) that high-quality printed matter be provided: optical density of the printed matter, sharpness of green, degree of bleeding between different adjacent colors in the case of multicolor printing, and the like. Interaction between the ink composition and the recording medium is a factor that greatly affects these items. In the particular case of plain paper, which is used on a large scale in offices and the like, these characteristics vary greatly with the production method, and the quality of printed matter sometimes decreases considerably for certain combinations of ink compositions and plain-paper recording media. This tendency is even more pronounced for recycled paper, whose frequency of use has been increasing in recent years. Specifically, there is a need for an ink composition that can produce the same high-quality printed matter even when printing is performed on a variety of types of plain paper, recycled paper, and the like.
The requirement (2) that rapid drying be ensured on the recording medium stems from the fact that no soiling should occur even when printed portions come into contact with each other immediately after recording, that no soiling or the like be caused by the movement of coloring materials between recording media even when recording is performed continuously and the printed matter is stacked, and the like. Among these, the first characteristic is important in the sense of having a direct effect on improving the recording rate. Rapid drying has an effect on phenomena such as the evaporation, penetration, and solidification of ink compositions on recording media, and conventional methods are also based on utilizing these phenomena. For example, a method in which the addition of a surfactant is limited to a value no greater than the critical micelle concentration thereof is proposed in Japanese Patent Application Laid-open No. S56-049771, a method for using a dialkylsulfosuccinate as a surfactant having a specific effect is proposed in Japanese Patent Application Laid-open No. 556-095961, and a method for using a surfactant that has an acetylene glycol backbone is proposed in U.S. Pat. No. 5,183,502. These methods are aimed at ensuring rapid drying by allowing the ink composition to rapidly penetrate the recording medium.
The items taken into account in a comprehensive manner in relation to the requirement (3) that high reliability be maintained during recording include not only the requirement that stable recording needs to be performed under regular service conditions, but also the requirements that the non-recording time (sometimes also referred to as “the decap time” or the like) from the moment the cap is removed until the moment a next recording is made needs to be extended; that stable recording without any plugging of the tips of ink-ejecting nozzles or the like needs to be performed even after the recording device has been allowed to stand in a hot, humid environment; and the like. These characteristics are greatly affected by the choice and compatibility of materials constituting the ink composition. Specifically, organic solvents with a low vapor pressure are conventionally used as constituent materials in order to minimize variations in ink compositions due to the evaporation of the materials in the compositions from the tips of ink-ejecting nozzles. Phase separation, deposition of insolubles, and other problems may also be reduced by taking into account the compatibility of the constituent materials.
The three major requirements described above are often in conflict with each other. For example, it is sometimes the case that the decap time decreases, plugging is apt to occur, and other problems are encountered when fast drying is emphasized and a material with a high evaporation rate is added to the ink composition as a constituent substance. It is also sometimes the case that the viscosity of an ink composition increases to render regular ejection unstable or that the quality of printed matter is reduced by some of the organic solvents used when plugging properties are emphasized and an organic solvent with a low evaporation rate or the like is added in a large amount.
In view of this, the present invention, which addresses such problems, is aimed at providing an inkjet ink that has exceptionally rapid penetration and allows printing characterized by minimal spreading to be performed, even without a particular provision of heating means, on plain paper, and particularly on recycled paper whose use has been growing in recent years.
Another object of the present invention is to provide an inkjet ink of high recording reliability that produces high-quality printed matter even when used to record information on various types of plain paper or recycled paper; has fast drying properties to prevent soiling from being caused by contact with fingers, other recording media, or the like even immediately after recording; and is less likely to create plugging or other defects.